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• lavc : clarify meaning of avctx.level option

  26 août 2023, par Stefano Sabatini

  lavc : clarify meaning of avctx.level option
  

  • [DH] doc/codecs.texi
  • [DH] libavcodec/avcodec.h
  • [DH] libavcodec/options_table.h

• Meaning of "ffprobe -v trace demo.mp4" output

  17 mai 2019, par bluesky

  The ’mdat box’ of Mp4 file may at the last of file. I want to know the position of ’mdat’ box using ’ffmpeg’ or ’ffprobe’.

  Mp4 consists of ’ftyp’, ’moov’ and ’mdat’ BOX. each BOX consists of "BoxHeader" and "BoxData". "BoxHeader" consists of "BoxSize(4Byte)", "BoxType(4Byte)", "BoxLargesize(8Byte, only have when box size exceeding the range of 4Byte expression, then the value of BoxSize is 1)".

  In program, you could first read 8 Byte and know the size of ’ftyp box’, then seek the size and read 8 Byte to know if the next box is ’moov box’. If not ’moov’, it shoud be ’mdat box’, then seek cross ’mdat box’ to find ’mdat box’...

  But I want to use ’ffprobe’ to find the position of ’moov’. I use ’ffprobe -v trace demo.mp4’, and output is like below

  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] Format mov,mp4,m4a,3gp,3g2,mj2 probed with size=2048 and score=100
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] type:'ftyp' parent:'root' sz: 28 8 41044500
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] ISO: File Type Major Brand: mp42
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] type:'moov' parent:'root' sz: 17943 36 41044500
  
  [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fc8fd000e00] type:'mvhd' parent:'moov' sz: 108 8 17935

  I want to know the meaning of type:'ftyp' parent:'root' sz: 28 8 41044500 :
  type:'ftyp' parent:'root'is easy to know, sz: 28 8 41044500 is really make me confused, I guess 28 is size of ftyp box,but the meaning of 8 41044500 is what ?

  Could you explain the meaning of sz: 28 8 41044500, and where could find the doc ?

• How to catch a FFMPEG exception with subprocess ?

  13 mai 2022, par Faindirnomainzein

  I'm doing some work on subtitles and some videos have 1 subtitle track, others have 2 subtitle tracks. For those that have 2, I use the 2nd one (index = 1). I'm trying to automate it with python.

  


  For files with with 2 subtitle tracks, I use :

  


  


  -vf "subtitles='file.mkv':si=1

  


  


  and for those with 1 subtitle track, I use :

  


  


  -vf "subtitles='file.mkv':si=0

  


  


  I'm using this code :

  


  for mkv in all_mkvs:
    try:
      subprocess.call(f'ffmpeg -i ... -vf "subtitles='file.mkv':si=1 ...')
    except:
      subprocess.call(f'ffmpeg -i ... -vf "subtitles='file.mkv':si=0 ...')


  


  But it doesn't seem to care about the exception and just ends the loop whenever it meets a file with 1 subtitle and gives me the error anyway.

  


  Press [q] to stop, [?] for help
[Parsed_subtitles_0 @ 0000011af4912880] Shaper: FriBidi 1.0.10 (SIMPLE) HarfBuzz-ng 2.7.2 (COMPLEX)
[Parsed_subtitles_0 @ 0000011af4912880] Unable to locate subtitle stream in ./test/349.mkv
[AVFilterGraph @ 0000011af623d880] Error initializing filter 'subtitles' with args './test/349.mkv:si=1'
Error reinitializing filters!
Failed to inject frame into filter network: Operation not permitted
Error while processing the decoded data for stream #0:3
Conversion failed!


  


  As you can see in the error message above, it says : "Error initializing filter .... si=1" because it should be si=0 in the case of that specific file, which is why I added the exception, but it doesn't seem to care about it.

  


  

  


  So I'm trying to catch that error and say "ok, in that case, let's do si=0 instead".